As conventional recordable disk-shape recording media, an MD (Mini Disc), a CD (Compact Disc)-R (Recordable), a CD-RW (ReWritable), a DVD (Digital Versatile Disc or Digital Video Disc)+RW (ReWritable), a DVD-R (Recordable), a DVD-RW (ReWritable), etc. are proposed. For formats of these disk-shape recording media, a groove recording format for recording in grooves is adopted.
For each format of an magneto-optical (MO) disk of ISO (International Organization for Standardization), a land recording format for recording in lands (between grooves) is proposed. In a DVD-RAM (Digital Video Disc-Random Access Memory) and the like, as one method for realizing higher density of an optical disk, a system (land and groove recording) for providing higher density by doubling the track density of the conventional one by recording in both grooves and between grooves (lands) is proposed. Here, the grooves refer to so-called guide grooves formed along recording tracks for mainly enabling tracking servo. The near part seen from an optical pickup is referred to as “groove”, and the far part is referred to as “land”. Note that the part between the grooves is referred to as “land”.
As shown in FIG. 1, in an optical recording medium having grooves formed, normally, tracking servo is performed using a push-pull signal. The push-pull signal refers to a difference signal, and is obtained by applying a light beam to the optical recording medium, detecting the light formed from the light beam reflected by the optical recording medium by two photo-detectors A and B disposed symmetrically relative to the track center, and calculating the difference (A−B) of outputs from these two photo-detectors A and B.
The reflected light amount of the light formed from the light beam reflected by the optical recording medium is detected as a sum (A+B) of the two photo-detectors. Here, a signal formed by detecting the reflected light amount of the light formed from the light beam reflected by the optical recording medium, that is, a sum signal of the outputs from the two photo-detectors A and B is a signal used for detecting how many tracks a spot of the light beam traverses when the spot moves, and generally referred to as “cross track signal (CTS)”.
In an MD or a CD-R, a ratio of “groove width/track pitch” is selected as on the order of 1/3 or 2/3 so that the push-pull signal and the CTS signal may be obtained sufficiently. That is, in the case of an MD, “groove width/track pitch”=1.1 μm/1.6 μm=69%, and, in the case of a CD-R, “groove width/track pitch”=0.5 μm/1.6 μm=31%
Furthermore, as a technology for improving linear recording density, DWDD (Domain Wall Displacement Detection) is proposed. This is one of magnetic domain enlargement and reproduction technologies used in a magneto-optical disk. For example, in Japanese Patent Application Laid-Open (JP-A) No. 6-290496, a technology relating to the DWDD system is disclosed.
FIG. 2 shows a partially enlarged sectional view of a magneto-optical disk disclosed in JP-A No. 6-290496. Reference numeral 71 denotes a substrate, reference numeral 72 denotes a dielectric layer, reference numeral 73 denotes a recording layer, and reference numeral 74 denotes a dielectric layer. Further, reference numeral 75 denotes a groove, and reference numeral 76 denotes a land.
The recording layer 73 is formed by sequentially laminating a first magnetic layer, a second magnetic layer, and a third magnetic layer. The first magnetic layer is consisted of a perpendicular magnetizing film having relatively smaller magnetic domain wall magnetic drag and larger magnetic domain wall mobility to the third magnetic layer at temperature close to ambient temperature, the second magnetic layer is consisted of a magnetic layer having lower Curie temperature than the first magnetic layer and the third magnetic layer, and the third magnetic layer is consisted of a perpendicular magnetizing film. Recording of data signals is performed by modulating the exterior magnetic field while applying a power laser beam that makes the third magnetic layer at equal to or more than Curie temperature with the medium being moved.
Further, JP-A Nos. 11-296910 and 2000-40259 propose pre-format by which recording density equal to the land and groove recording is achieved, depths of two grooves are suitably changed, the two grooves having different depths are arranged adjacently, and sufficient CTS signal amplitude and push-pull signal amplitude can be obtained even when the cutoff frequency is exceeded. This pre-format realizes stable tracking servo in a track pitch exceeding the cutoff frequency. In the pre-format, deep grooves and shallow grooves are adjacently arranged, and the interval of the deep grooves (or shallow grooves) is a track period (1.0 μm) and the interval of the deep groove and the shallow groove is a track pitch (0.5 μm). Two lands (track A, track B) on both ends of the shallow groove sandwiched by deep grooves are a recording area. Therefore, the track density in the pre-format is high density twice the conventional one, that is, equal to the recording density of the land and groove recording.
That is, the track density of the land and groove recording is about twice the conventional one, and, in the pre-format, the recording area is also two lands (track A, track B) on both ends of the shallow groove, and equal to the track density of the land and groove recording. Therefore, it has been difficult to make the track density into higher density equal to or more than twice the conventional one.
Further, in the land and groove recording, the groove width and the land width are substantially the same. When the groove width and the land width are substantially the same, the push-pull signal provides a sufficient signal amount at the maximum, however, the signal amount of the CTS signal becomes insufficient, as shown in FIG. 1. In a normal disk reproducing apparatus, for seek operation, about 6% to 7% is required for a signal amount of a signal for counting the number of tracks, and a signal amount of about 14% is required for a detection signal for tracking servo. Here, the signal amount is defined with a signal obtained on a surface on which no groove or pit is formed (so-called dummy surface) as 100%.
As described above, if the signal amount of the CTS signal is insufficient, at the time of seeking operation for moving toward a target address at high speed, there is a problem in seeking by the CTS signal because the number of traversed tracks can not be detected accurately from the CTS signal. Needless to add, it is impossible that tracking servo is applied with a CTS signal in a small signal amount.
Further, in order to obtain these push-pull signal and CTS signal having required signal amounts, the spatial frequency of the track pitch is needed to be made into about ½ to ⅔ of the cutoff frequency of the optical pickup of the reproducing apparatus. Here, the cutoff frequency refers to a frequency at which the reproduction signal amplitude becomes nearly zero, and is expressed by 2NA/λ assuming that the wavelength of a laser beam used for data reproduction is λ, and the numerical aperture of an objective lens is NA.
Therefore, an object of the invention is to provide an optical recording medium, a master for optical recording medium manufacture, a recording/reproducing apparatus, and a recording/reproducing method by which higher track density is realized, and, even when the groove width and the land width are substantially the same, the signal amounts of the push-pull signal and the CTS signal can be obtained sufficiently.